Ductility contrast induced by silicification in pelitic schist of the Ryoke metamorphic belt, Japan Tayyaba Mateen a, * , Hiroshi Yamamoto a , Hafiz Ur Rehman a , Masaru Terabayashi b a Graduate School of Science and Engineering, Kagoshima University, Kagoshima, 890-0065, Japan b Department of Safety Systems Construction Engineering, Kagawa University, Takamatsu, 761-0396, Japan article info Article history: Received 18 November 2014 Received in revised form 16 July 2015 Accepted 26 August 2015 Available online 1 September 2015 Keywords: Ryoke metamorphic belt Pelitic schist Silicification Quartz veins Bright spot abstract Contrasting ductility is recognized in the rocks of Cretaceous Ryoke metamorphic belt in Iwakuni area, southwest Japan. Pelitic schist is ubiquitous in the region and differences in mineral assemblages mark increase in metamorphic grade. The area has been graded as chlorite-biotite zone in the north pro- gressing into biotite- and muscovite-cordierite zones in the south. Pelitic schist near the boundary be- tween the biotite- and muscovite-cordierite zones has undergone partial silicification to form whitish silicified schist layers which contain two types of quartz veins: those parallel to foliation in the host rock are called schistosity-concordant veins, and those inclined to host rock foliation, schistosity-discordant veins. In this study we examined the quartz structure in the silicified schist and in both types of veins to understand the ductility contrast induced by the silicification process. Crystallographic orientations of quartz in the veins and silicified schist rocks were studied using the Scanning Electron Microscopy (SEM) based Electron Back Scatter Diffraction (EBSD) technique. Quartz c-axis orientations in the silicified schist are nearly random, demonstrating an absence of post-silicification ductile deformation. Quartz grains in the schistosity-concordant veins have preferred c-axis orientations perpendicular to the schistosity indicating ductile shortening. In contrast, schistosity-discordant veins display distinct quartz c-axis fabric than that found in the schistosity-concordant veins. This is because the two types of host rocks exhibit a difference in ductility during deformation. The presence of deformed quartz veins in the undeformed silicified schist indicates transformation of the ductile pelitic schist into the brittle silicified schist at mid- crustal levels where these rocks originate, hence forming contrasting rock layers. Schistosity-concordant veins in the biotite-rich pelitic schist deformed with its host rock in a ductile manner while the schistosity-discordant veins in the neighboring silicified schist were left intact. Silicification of the pelitic schist may have been caused by the silica-rich geofluids produced by subsurface processes. Geofluids responsible for the occurrence of such mechanically contrasting layers mark an increase in seismic reflectivity at mid-crustal depths and may be potential reflectors of seismic waves giving rise to the so- called “bright spots”. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Brittle deformation such as faulting dominates the Earth's crust progressing into ductile deformation in deeper regions where temperatures and lithostatic pressures are higher (e.g. Passchier and Trouw, 2005). Ductile deformation may be preserved as rock microstructures that can be used to delineate a rock's thermo- mechanical history. Crystallographic preferred orientations (CPO's) of quartz (e.g. Lister and Hobbs, 1980; Schmid and Casey, 1986) are a popular type of microstructures that provide consid- erable information on deformation of quartz-rich rocks. A variety of sub-surface processes including hydrothermal alteration associated with subduction (e.g. Bebout and Barton, 1989; Breeding and Ague, 2002; Fagereng and Harris, 2014) produce fluids which interact with rocks and may influence the rock physical properties (e.g. Chigira and Watanabe, 1994) and subsequent formation of rock microstructures. Mechanical contrast in the rocks arises due to chemical differences in rock layers induced by underlying fluids believed to exist under high pressures in the crust (e.g. Sibson, * Corresponding author. E-mail addresses: k9977115@kadai.jp (T. Mateen), hyam@sci.kagoshim-u.ac.jp (H. Yamamoto), hafiz@sci.kagoshima-u.ac.jp (H.U. Rehman), tera@eng.kagawa-u. ac.jp (M. Terabayashi). Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg http://dx.doi.org/10.1016/j.jsg.2015.08.009 0191-8141/© 2015 Elsevier Ltd. All rights reserved. Journal of Structural Geology 80 (2015) 38e46